The pinning strength and upper critical fields of magnetic and nonmagnetic artificial pinning centers in Nb47w/oTi wires

Nb47^w/oTi wire samples were fabricated with copper 12Vol%, 6Ni/6Cu, and 8Ni/4Cu artificial pinning centers (APC). The optimum diameter of the "island" pins was 10 to 12 nm at final wire size. Low temperature heat treatment of sample wires was performed at final diameters to form alloy pins. The upper critical field, Hc₂, irreversibility field, H^*, and the critical current density, J_c, were measured at 4.2 K. Magnetization measurements showed highest Hc₂ of 11 T and H^* of 9.8 T in the best APC wire. In addition, significant improvement in J_c was obtained at all fields above 5 T compared to past APC designs. At 5 T, J_cs up to 5034 A/mm² were obtained by transport measurements. Field emission scanning electron microscopy (FESEM) of the pins at near final wire diameters suggest a new feature. The possible presence of voids formed on the outside of each pin as a result of nickel and copper inter-diffusion (Kirkendall effect) from low temperature heat treatment.